Sunlight-induced skin cancer is the most prevalent cancer in the United States, and UVB is primarily responsible for these cancers. Earlier studies indicated that oral or topical administration of caffeine inhibits UVB-induced skin carcinogenesis in mice by enhancing apoptosis selectively in UVB-treated epidermis and in epidermal tumors but not in normal epidermis. These effects can occur by a p53 independent mechanism. Identification of mechanisms for the in vivo apoptotic effect of caffeine in UVB-treated epidermis and in UVB-induced tumors as well as the initiation of translational studies in humans are major goals of the present proposal. We plan to pursue the following specific aims: 1. Determine the effects of topical application of caffeine immediately after a single irradiation with UVB on the time course for UVB-induced apoptosis and for UVB-induced activation of the ATR/Chk1 signal transduction pathway in the epidermis of wild type mice, p53 knockout mice, ATR and Chk1 dominant negative mice and Chk1 haploinsufficient mice. The possibility that caffeine enhances the inappropriate initiation of mitosis early in the cell cycle will also be determined. 2. Determine the effects of topically applied caffeine on apoptosis and the ATR/Chk1 signal transduction pathway as well as the possibility of inappropriate early initiation of mitosis in UVB-induced tumors and in areas of the epidermis away from tumors. 3. Initiate a pilot study to determine the effects of topical application of caffeine immediately after a single irradiation with UVB on UVB-induced apoptosis as well as the levels of wild type p53 and the ATR/Chk1 signal transduction pathway in the epidermis of human skin. Since caffeine is ingested by large segments of the population in coffee, tea, soft drinks and chocolate, research on mechanisms of the potential anticancer effects of caffeine may have broad human significance for cancer prevention. The proposal also includes the first human studies on a path that may lead to a novel way of preventing sunlight-induced skin cancer in humans.